Continuous process for the preparation of pentafluoroethyl iodide

ABSTRACT

Continuous process for the preparation of pentafluoroethyl iodide from iodine, iodine pentafluoride and tetrafluoroethylene, which comprises dissolving the iodine in iodine pentafluoride and continuously feeding the resulting solution to a vertical bubble column ( 1 ), which is filled with iodine pentafluoride and at its lower end has a feed line for tetrafluoroethylene, where the reaction zone is maintained at from 85 to 95° C., preferably 90° C., and the pentafluoroethyl iodide formed escapes in gaseous form at the top of the reactor ( 1 ), is liquefied in a downstream cooled condenser and is drawn off into storage vessels.

This application is a 371 of PCT/EP/98/03909 filed Jun. 26, 1998.

DESCRIPTION

Pentafluoroethyl iodide is a valuable synthon for introducing thepentafluoroethyl group into organic compounds which, for example, haveherbicidal action. However, pentafluoroethyl iodide has achievedgreatest significance as a telogen in socalled telomerization withtetrafluoroethylene and/or hexafluoropropene. The perfluoroalkyl iodideswhich form in the process are important starting materials for numeroussyntheses, which give active ingredients having strongly markedhydrophobic and oleophobic properties.

Pentafluoroethyl iodide is prepared predominantly in accordance withequation (I) below from tetrafluoroethylene, iodine pentafluoride andiodine:

5CF₂=CF₂+IF₅+2I₂→5CF₃CF₂I  (I)

A corresponding process is described, for example, in U.S. Pat. No.3,406,214. It involved reacting a mixture of iodine pentafluoride and10% by weight of iodine with gaseous tetrafluoroethylene at from 60 to80° C. A conversion of 40% by weight and a yield of 90% by weight ofpentafluoroethyl iodide were achieved.

There has been no lack of attempts to accelerate the preparation processaccording to the above equation (I) using catalysts, in particular usingcatalysts from the group of Lewis acid metal compounds, for exampleTiCl₄, ZrCl₄ or VF₅ (see DE-C-20 33 755). However, a disadvantage ofthis and similar processes is the increased rate of corrosion on thestainless steel apparatuses which are normally used.

In view of the potential hazard which originates in particular from theextremely reactive and toxic iodine pentafluoride, a preparation plantfor pentafluoroethyl iodide by the process according to the equation (I)must satisfy particular requirements with regard to freedom from leaksand must have as few moving parts as possible. The addition of theiodine in liquefied form (by melting under pressure) has proven not tobe recommended because of the corrosivity toward metallic materials;even nickel-based alloys are not completely durable during the storageof liquid iodine. Moreover, the exact metered addition of thepressurized, liquid, about 125° C.-hot iodine to the iodinepentafluoride, which is present in the reactor and has a temperature ofabout 90° C., has proven to be problematical.

There was therefore a great need for a safe and simple process for thepreparation of pentafluoroethyl iodide according to equation (I). Theobject according to the invention is achieved. essentially by continuousdissolution of solid iodine in iodine pentafluoride. The flow diagram ofthe process is shown in the Figure. In detail, the process consists ofthe following steps: A vertical bubble column 1, which is equipped witha level regulator, is filled with IF₅ and heated to from 85 to 95° C.,preferably 90° C. A defined amount of crystalline iodine is transferredfrom an iodine transportation container using a lifting and tiltingdevice 4 into the charge transfer tube 5 and from there is passedbatchwise to the iodine dissolution vessel 3 filled with IF₅. “Weakened”IF₅ is continuously passed into the iodine dissolution vessel 3 from thebubble column 1 using the circulation pump 2. The iodine dissolutionvessel 3 is designed such that the filling with solid iodine and theintroduction of the “weakened” IF₅ is separated from the overflow of theIF₅ concentrated with iodine by a calming zone. The IF₅ concentratedwith iodine to the solution equilibrium is allowed to flow continuouslyvia an overflow into the reactor 1, while tetrafluoroethylene issimultaneously introduced at the foot of the reactor 1 at the rate atwhich it is consumed by the reaction. Pure pentafluoroethyl iodideescapes at the top of the reactor 1, is liquefied in a cooled condenserand collected in a storage tank. The IF₅ consumed in the reaction isreplenished, by means of a level regulator in the bubble column 1, fromthe storage container 6 via the pump 2 into the IF₅ cycle. The amount ofsolid iodine in the dissolution vessel 3 is determined by means of aradioactive level measurement. The solubility of iodine in iodinepentafluoride is 5.2% by weight at 20° C. and from 9 to 10% by weight atfrom 85 to 95° C.

The process according to the invention has the following advantages:

a) liquid iodine is not used. This avoids the serious problems whichresult from working with liquid iodine both in terms of processtechnology and with regard to industrial safety.

b) Apparatus, pipelines and fixtures can be made of customarychromium-nickel steels, for example steel 1.4571. This meanssignificantly lower capital costs compared to liquid iodine processes.

c) The yield of pentafluoroethyl iodide is, in continuous operation,from 97 to 98% of the theoretical yield, based on IF₅. This correspondsto the best yield which has been achieved using Lewis acids such as SbF₃or PCl₅ as catalysts (see JP-A-60/023333) . It is self-evident that agreat advantage of the process according to the invention is that it canbe carried out without Lewis acids or other catalysts.

What is claimed is:
 1. A continuous process for the preparation of purepentafluoroethyl iodide from iodine, iodine pertafluoride andtetrafluoroethylene, which comprises dissolving the iodine in a mixturewhich is separated from the lower end of a vertical bubble column (1)and continuously feeding the resulting solution to said vertical bubblecolumn (1), which is equipped with a level regulator, is filled withiodine pentafluoride and at its lower end has a controllable feed linefor tetrafluoroethylene, where the reaction zone is maintained from 85to 95° C., and the pentafluoroethyl iodide formed escapes in gaseousform at the top of the reactor (1), is liquefied in a downstream cooledcondenser and is drawn off into storage vessels.
 2. The process asclaimed in claim 1, wherein crystalline iodine is fed from suitablestorage containers by means of a lifting and tilting device (4) to acharge transfer tube (5), from which the batchwise addition of theiodine into a dissolution vessel (3) takes place, where the iodine isdissolved in iodine pentafluoride.
 3. The process as claimed in claim 1wherein iodine pentafluoride is fed continuously at the rate at which itis consumed from a storage container (6) using a pump (2) to the circuitwhich leads via the iodine dissolution vessel (3) to the reactor (1). 4.The process as claimed in claim 2, wherein iodine pentafluoride is fedcontinuously at the rate at which it is consumed from a storagecontainer (6) using a pump (2) to the circuit, which leads via theiodine dissolution vessel (3) to the reactor (1).
 5. The process asclaimed in claim 1 wherein the reaction zone is maintained at 90° C.